1. Field of the Invention
The present invention relates to an optical recording medium for optically recording and reproducing information.
2. Related Background Art
In general, optical recording mediums, for example, optical discs and optical cards can record information in a high density by forming optically detectable pits of minute size, for example, of about 1 .mu.m on a thin recording layer provided on a substrate having grooves of spiral, circular or linear form.
To write information in such an optical recording medium, a focused laser beam is scanned on the surface of a laser beam sensitive layer, whereby pits are formed only on the surface on which the laser beam was irradiated and the information is recorded The laser-beam-sensitive layer can absorb energy to form optically detectable pits thereon. For example, in a certain heat mode recording system, the laser-beam-sensitive layer absorbs a heat energy to form minute concaves. i.e., pits, on those energy-absorbed parts by evaporation or fusion. In another heat mode system, the absorption of the energy of the irradiated laser beam can form pits having an optically detectable density difference on those parts.
The information recorded on the optical recording medium in this manner can be detected by reading optical changes between the part on which the pits are formed and that part on which the pits are not formed.
Hitherto known laser beam-sensitive layers or recording layers used in such optical recording mediums include those in which inorganic materials are chiefly used, for example, those in which bismuth thin films, tellurium oxide thin films, chalcogenite type amorphous glass films or metal thin films such as aluminum or gold thin films are used as disclosed in Japanese Patent Publication No. 40479/1871, Japanese Patent Laid-open No. 27395/1981, etc. These thin films, however, have been disadvantageous such that they involve poor storage stability, low resolution power, low recording density, high production cost, etc.
Recently, also proposed in Japanese Patent Laid-open No. 187948/1985, Japanese Patent Laid-open No. 205841/1988, etc. is to use in the recording layer an organic coloring matter thin film whose physical properties can be changed by light of a relatively long wavelength, for example, of 780 nm or more. In this organic coloring matter thin film, the pits can be formed by use of a semiconductor laser beam having an oscillation wavelength, for example, of around 830 nm, thus eliminating the drawbacks possessed by the above mentioned thin films chiefly employing the inorganic materials. In general, however, organic coloring matters having absorption characteristics on the side of the long wavelength has the problem such that they have a low stability to heat and light.
An organic coloring matter that can solve the above problem is disclosed in Japanese Patent Laid-open No. 181690/1983. In the invention disclosed therein, the organic coloring matter has a great absorption band to a wavelength (750 to 850 nm) of conventional semiconductor lasers. Recent years, however, a semiconductor laser that can oscillate light of a shorter wavelength, for example, of a visible light region has been developed. Thus, an organic coloring matter having a great absorption band even to the light from such a semiconductor laser has been sought after.